Lindsay Kujawa just turned away from her son Ronin for “maybe five seconds” and the toddler tumbled into the water at a pool party.

“I just turned … and he had jumped into the spa and the jets whirled him to the other side,” Kujawa, who is from San Diego, told ABC’s "Good Morning America" today.

Kajawa said she pulled him out after about 20 seconds and Ronin seemed unscathed. "I'm like he's okay,” she said. “He's not blue, he's choking up the water, he seems fine."

But, he was not out of the woods. About an hour later, Ronin started to cough and became lethargic, and by the time Kujawa got him to the emergency room, the toddler was diagnosed with a little known, but potentially fatal condition -- sometimes called secondary drowning.

“This case is not a total surprise and it can happen, but not in the majority of cases,” said Dr. Paul Pepe, chair of emergency medicine at University of Texas Southwestern Medical Center in Dallas, who has conducted numerous studies on drowning in Texas and in Florida, where rates are high.

“It’s not a common scenario,” he told ABC News. “But it does occur."

Pepe said even children who are resuscitated and rushed to the emergency room after falling in a pool can develop pulmonary edema in the first three or four hours after taking in water.

USA Swimming and the Pool Safely campaign just today released numbers of drowning deaths in pools and spas in the United States. They say there have been 95 in children under 14 this year through May 31; of those cases, 74 involving children younger than 5. But they do not track deaths from secondary drowning.

“We don’t use that phrase,” said Ellyn Pollack, spokesman. “Basically, they die from drowning. Whether it’s two days later or on the scene, it’s drowning.”

Drowning is the number one cause of deaths among children under the age of 5, according to Pepe. Two-thirds of all drownings occur in the summer months and most occur in residential pools, not at the beach.

“You can be a very conscientious mother – it takes only a blink of an eye to get water in the lungs to cause drowning," Pepe said.

And CPR poolside can make all the difference between death and survival. Without it, there is only a 5 percent chance of resuscitation with no neurological damage, he said. “The sooner you can get to the child, the better.”

Pepe was part of a decade-long study of drowning in Houston and Florida in the 1990s, and was chairman of the Resuscitation Task Force for the World Congress for Drowning in 2002. “We looked at these cases very carefully.”

In secondary drowning, the pool water can damage or “wash out” the surfactant or lining of the lungs, said Pepe. “You can have kind of a lung failure. It's complicated."

Negative pressure in the lungs can cause pulmonary edema or swelling after being pulled from the pool.

“In many cases, these kids look great at first, even after CPR,” Pepe said. “We don’t really know why it happens but water comes into the lungs and within minutes it is actually starting to be absorbed into the body itself. If you have a big load of water, chemical transfers occur in the first few minutes that do things that can be lethal.”

Ronin's mother said he stayed in the hospital for 24 hours, but is doing well now. Doctors warn parents to keep an eye on a child, watching for difficulty breathing, coughing or lethargy, if they have taken in water in an incident like Ronin’s.

Parents should take their child to the emergency room immediately. And other experts say, teach your child to swim, and if you have a pool, keep the gates locked. And when poolside with your child, never take your eyes off them.

As for Kujawa, she says, “I now know that you literally cannot turn your back for literally five seconds."

"I am forever changed since this happened," she writes on her blog. "I will not let this define me but you can guarantee that I will be doing things much differently from here on out. It was a huge wake-up call. And it really taught me that yes...in just a few seconds your life can change forever. I got a too close taste of what that really can be like."